Trunked radio communication systems are known. Such systems typically allocate communication resources to communication units upon a perception of a need for service by a resource controller.
Transceivers are provided at communication system base sites to receive signals from communication units and to re-transmit the signals for the benefit of any target units present within the service coverage area of the base site. Some transceivers present at the base site are also capable of, upon occasion, providing an interconnect between the communication unit and a local public switched telephone network (PSTN).
PSTNs are also known. PSTNs offer communication services primarily over wireline based communication facilities. Services offered include basic telephone service as well as a number of auxiliary services offered independently or through integrated services digital networks (ISDNs). ISDNs, as is known, offer a variety of voice and data services.
Key to accessing ISDN services is the transmission of control characters into the PSTN system. Access to an ISDN system, and the type of service provided, is dependent upon providing the proper control words under protocols as follows: Q.931 for voice and data service, Q.932 for supplementary services, and X.25 for packet mode data servicing (for an detailed description of control of an ISDN system, refer to Bellcore--Bell Communication Research Pub. No. TR-TSY-000268 (Issue #3, May 1989)).
In the past, creation of control characters and protocols for accessing ISDN services has been under the control of a computer (or data processing equipment with an AT&T SESS switch or Northern Telecom DMS switch) or other data processing equipment connected to the PSTN network and typically under the manual control of an operator. Because of the importance of ISDN services a need exists for access to ISDN on a PSTN from other communication systems.